[SystemZ] Fix RNSBG bug introduced by r197802

The zext handling added in r197802 wasn't right for RNSBG.  This patch
restricts it to ROSBG, RXSBG and RISBG.  (The tests for RISBG were added
in r197802 since RISBG was the motivating example.)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198862 91177308-0d34-0410-b5e6-96231b3b80d8